Microfluidic Transfer Pins

ABSTRACT

A liquid dispenser for a microfluidic assay system is described. The dispenser includes at least one transfer pin for transferring a microfluidic sample of liquid to a target receptacle. A pin tip at one end of the transfer pin is structured to cooperate with an opening in the target receptacle. The tip uses a high voltage potential to transfer the sample from the pin to the receptacle.

FIELD OF THE INVENTION

The invention generally relates to techniques for assaying small volumesof liquid, and more specifically to physical transfer of a small volumeinto a storage medium.

BACKGROUND ART

Techniques are rapidly developing for parallel performance of a largenumber of chemical and biological assays and synthesis operations. Oneapproach uses a nanotiter plate having a high density platen ofthrough-hole wells with hydrophilic interiors and openings surrounded byhydrophobic material. This is described, for example, in U.S. Pat. No.6,387,331 and U.S. Patent Application 20020094533, the contents of whichare incorporated herein by reference. One specific commercial example ofa nanotiter plate system is the Living Chip™ made by Biotrove, Inc. ofCambridge, Mass. Nanotiter plate technology relies on the ability tohandle very small volumes of fluid samples, typically, 100 nanoliters orless. The various considerations taken into account in handling suchsmall liquid samples are known as microfluidics.

Transferring of large collections of fluids such as libraries of smallmolecule drug candidates, cells, probe molecules (e.g., oligomers),and/or tissue samples stored in older style 96- or 384-well plates intomore efficient high density arrays of microfluidic receptacles such as ananotiter plate can consume one or more hours, during which time samplesmay evaporate, degrade or become contaminated. It is thereforeadvantageous to submerse the array in a bath of immiscible fluid. Thefluid is ideally electrically insulating, non-conductive andnonflammable, with a relative permittivity >1. One class of fluids thatserves this purpose is perfluorinated hydrocarbons, such aperfluorodecalin, perfluorooctane, perfluoropentane, longer chainedperfluorocarbons or mixed populations of perfluorocarbons. Hydrocarbonsor silicone fluids would also work but are flammable and tend to extractcompounds from the sample.

A microfluidic volume of a liquid sample may be loaded into a targetreceptacle by various means. One established method for transferring aliquid sample to a surface or to another liquid uses a transfer pinloaded with the sample liquid. For example, pins or arrays of pins aretypically used to spot DNA samples onto glass slides for hybridizationanalysis. Pins have also been used to transfer liquids such as drugcandidates between microplates or onto gels (one such gel system isbeing developed by Discovery Partners, San Diego, Calif.). Many pintypes are commercially available, of various geometries and deliveryvolumes. V&P Scientific of San Diego, Calif. makes slotted, grooved,cross-hatched, and other novel-geometry pins. The Stealth Pin by ArrayItis capable of delivering hundreds of spots in succession from one sampleuptake, with delivery volumes of 0.5 nL to 2.5 nL. Majer PrecisionEngineering sells pins having tapered tips and slots such as theMicroQuil 2000.

U.S. Pat. No. 6,149,815 describes an approach for dispensing liquidsamples electrokinetically. A complex apparatus positions a receiverreservoir and a non-conducting liquid dispenser between a ground plateand a high voltage plate, neither plate being electrically connected toa sample. An accurate volume of liquid sample is transferred from thedispenser to the receiver reservoir by precisely controlling the timethat a high voltage is applied to the dispenser, the longer the voltageis applied, the greater the volume of sample transferred, and viceversa. As shown in FIG. 1 of the '815 patent, it is important to providean insulating gap between the electrically charged dispenser and theelectrically grounded receiver reservoir. Moreover, the '815 patentapproach requires determining by visual observation the relationshipbetween time, voltage, and volume of liquid transferred. Nonetheless,the '815 patent does suggest that high voltage electric potential may beuseful for transferring liquid samples from a loaded transfer pin.

SUMMARY OF THE INVENTION

A representative embodiment of the present invention includes a liquiddispenser for a microfluidic assay systems including systems forarraying samples for storage, screening and synthesis. The dispenserincludes at least one transfer pin for transferring a microfluidicsample of liquid to a target receptacle. A pin tip at one end of thetransfer pin is structured to cooperate with an opening in the targetreceptacle. The tip uses a high voltage potential to transfer the samplefrom the pin to the receptacle.

In a further embodiment, the target receptacle is one of an array ofthrough-holes wells or closed-end wells in a platen. The targetreceptacle may have hydrophilic walls that attract the sample. Thetarget receptacle may have an opening surrounded by hydrophobicmaterial. The target receptacle may be filled with a porous hydrophilicmaterial. A transfer pin array may include multiple transfer pins fortransferring multiple samples to corresponding target receptacles.Individual transfer pins in the array may be individually actuable, aswould be useful for producing patterns or layered patterns of samples.Typically the spacing of pins in the array will match a subset of asource array such as a 384 well microtiter plate as well as the spacingof the receptacle array. At least one transfer pin in the array may beindependently positionable to align the at least one independentlypositionable pin with respect to the opening of a target receptacle.Positioning systems are typically capable of accurate movement in atleast the x, y and z co-ordinates. Individual transfer pins in the arraymay be free floating or spring loaded.

In various embodiments, the microfluidic sample may be from 0.2 to 100nanoliters. The transfer pin may have a diameter greater than theopening of the target receptacle. The sample may be a polar liquid suchas aqueous, DMSO, dimethylformamide (DMF), or acetonitrile solutions.The high voltage potential may be between 100V and 5 kV. The at leastone transfer pin may be able to dispense multiple samples withoutreplenishment.

In a further embodiment, a voltage control module controls when the highvoltage potential is applied to and removed from the pin tip. Thevoltage control module may operate to apply the high voltage potentialto the pin tip before or after the transfer pin is positioned at thetarget receptacle, and to remove the high voltage potential before orafter the transfer pin is moved away from the target receptacle. Thevoltage control module may include a resistor network and/or acontrollable switch in series with the transfer pin.

Embodiments of the present invention also include a method for use indispensing a microfluidic sample of a liquid. The method includesproviding at least one transfer pin for transferring a microfluidicsample of liquid to a target receptacle. One end of the transfer pin mayhave a pin tip structured to cooperate with an opening in the targetreceptacle. Voltage is applied between the transfer pin and the targetreceptacle for transferring the sample from the at least one transferpin to the target receptacle.

In such an embodiment, the target receptacle may be a through-hole wellor a closed-end well in a platen array. The target receptacle also mayinclude hydrophilic walls that attract the sample and/or an openingsurrounded by hydrophobic material. The voltage may be applied to eitherthe transfer pin or the target receptacle.

The method may also include providing a transfer pin array includingmultiple transfer pins for transferring multiple samples tocorresponding multiple target receptacles. Individual transfer pins inthe array may be individually-actuable, either sequentially or inparallel. At least one transfer pin in the array may be independentlypositionable for alignment with respect to the opening of a targetreceptacle. Individual transfer pins in the array may be free floatingor spring loaded.

In such a method, the microfluidic sample may be from 0.2 to 100nanoliters. The transfer pin may have a diameter greater than theopening of the target receptacle. The sample may be a polar liquid suchas aqueous, DMSO, dimethylformamide (DMF), or acetonitrile solutions.The high voltage potential may be between 100V and 5 kV. The highvoltage potential may be applied before or after the transfer pin ispositioned at the target receptacle, and removed after the transfer pinis moved away from the target receptacle. The controlling step may use aresistor network and/or a controllable switch in series with thetransfer pin. The at least one transfer pin may be able to dispensemultiple samples without replenishment.

The method may further include applying evaporation control measures tothe target receptacle. This may include immersing the target receptaclein an immiscible liquid such as a perfluorinated hydrocarbon.Alternatively, or in addition, the evaporation control measures mayinclude at least one of humidity control, fluid pressure, and receptaclecooling.

The method may also further include positioning the transfer pin indirect contact with target receptacle, or positioning the transfer pinnear the target receptacle without direct contact. The method may alsoinclude sequentially transferring multiple samples to the targetreceptacle to produce a layered pattern of samples.

Another embodiment of the present invention includes a microfluidicassay system. The system includes at least one liquid sample storagedevice including multiple storage receptacles, a microfluidic dispenser,and a dispenser positioning module. The microfluidic dispenser has ahigh voltage supply that develops a high voltage potential; at least onetransfer pin for transferring a microfluidic sample of liquid to atarget storage receptacle, one end of the transfer pin having a pin tipstructured to cooperate with an opening in the target storagereceptacle; and a voltage controller for applying the high voltagepotential from the high voltage supply between the transfer pin and thetarget storage receptacle for transferring the sample from the at leastone transfer pin to the target storage receptacle. The dispenserpositioning module positions the liquid dispenser to enable the transferpin to cooperate with the target receptacle for transferring the sample.

In a further such embodiment, the storage device may be a platen arrayof through-holes or wells. The voltage may be applied to the transferpin or to the target storage receptacle. The target storage receptaclemay include hydrophilic walls that attract the sample and/or an openingsurrounded by hydrophobic material. The liquid dispenser may alsoinclude a transfer pin array including multiple transfer pins fortransferring multiple samples to corresponding multiple target storagereceptacles. Transfer pins in the array may be individually actuable,either sequentially or in parallel. At least one transfer pin in thearray may be independently positionable for alignment with respect tothe opening of a target storage receptacle. Individual transfer pins inthe array also may be free floating or spring loaded.

In such a system, the microfluidic sample may be from 0.2 to 100nanoliters. The transfer pin may have a diameter greater than theopening of the target storage receptacle. The sample may be a polarliquid such as aqueous, DMSO, dimethylformamide (DMF), or acetonitrilesolutions. The high voltage potential may be between 100V and 5 kV.

The voltage controller may apply the high voltage potential to the pintip before or after the transfer pin is positioned at the target storagereceptacle, and removes the high voltage potential after the transferpin is moved away from the target storage receptacle. The voltagecontroller also may use a resistor network and/or a controllable switchin series with the transfer pin.

In a system, the storage device may use evaporation control measures tocontrol evaporation of samples from the storage receptacles. This mayinclude immersing the storage receptacles in an immiscible liquid suchas a perfluorinated hydrocarbon and/or at least one of humidity control,fluid pressure, and receptacle cooling.

The positioning module may position the dispenser so that the at leastone transfer pin makes direct contact with target storage receptacle fortransferring the sample, or so that the at least one transfer pin isnear the target storage receptacle without direct contact fortransferring the sample.

The liquid dispenser may operate to sequentially transfer multiplesamples to the target storage receptacle to produce a layered pattern ofsamples. The at least one transfer pin may be able to dispense multiplesamples without replenishment.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more readily understood by reference tothe following detailed description taken with the accompanying drawings,in which:

FIG. 1 shows a cut away view of a nanotiter plate having one of itsthrough wells being loaded by a transfer pin bearing a liquid sampleaccording to one embodiment of the present invention.

FIG. 2 shows an elevated side view of an array of transfer pinsaccording to one embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Various embodiments of the present invention are directed to applying ahigh voltage to one or more transfer pins for transferring amicrofluidic volume of a liquid sample to a suitable target receptacle.The target storage receptacle typically will have an affinity for thesample, and could be a flat surface; a surface with indentations, closeended wells, or pores; a membrane or filter; a gel; or a platen withclose-ended wells or through-hole wells. In one specific embodiment, thetarget receptacle is one or more wells in an array of through-hole wellsas part of a parallel and/or series sample transfer process. In otherembodiments, the target storage receptacle may be a hydrophilic spot ordivot in a hydrophobic background. Such an environment may beestablished on a coated glass slide such as the ones available from ErieScientific of Portsmouth, N.H.

FIG. 1 shows a cut away view of a nanotiter plate having one of itsthrough-hole wells being loaded by a transfer pin bearing a liquidsample according to one embodiment of the present invention. Platen 10contains a large number of through-hole wells 12 that traverse theplaten 10 from one planar surface 14 to the other opposing planarsurface (not shown). The platen 10 is may be from 0.1 mm to more than 10mm thick; for example, around 0.3 to 1.52 mm thick, and commonly 0.5 mm.The thickness of platen 10 is also the length of the through-hole wells12 when they are oriented perpendicularly to planar surface 14. Thelength and volume of the wells 12 can be increased somewhat by orientingthem at an angle to surface 14.

Typical microfluidic volumes of the through-hole wells 12 could be from0.1 picoliter to 1 microliter, with common volumes in the range of0.2-100 nanoliters. Capillary action or surface tension of the liquidsamples may be used to load the wells 12. To enhance the drawing powerof the wells 12, the target area of the receptacle, interior walls 42,may have a hydrophilic surface that attracts a liquid sample.Alternatively, the wells 12 may contain a porous hydrophilic materialthat attracts a liquid sample. To prevent cross-contamination(crosstalk), the exterior planar surfaces 14 of platen 10 and a layer ofmaterial 40 around the openings of wells 12 may be of a hydrophobicmaterial. Thus, each well 12 has an interior hydrophilic region boundedat either end by a hydrophobic region.

In some systems, the well 12 may be submersed in an immiscible,non-conducting liquid such as perfluorinated hydrocarbon, hydrocarbon,or silicone fluid. An immiscible liquid prevents evaporation of samplesfrom the wells 12 and further protects the samples fromcross-communication. Of course, other evaporative control measures mayalso be useful, including without limitation, humidity control, fluidpressure, platen cooling, etc.

Transfer pin 20 is generally dowel-shaped, made of stainless steel,titanium, or other durable material, with a flat, rounded, tapered, orcupped tip. Typically, although not necessarily, the diameter of thetransfer pin 20 is greater than the diameter of the wells 12 in order tohave more rigidity in the pin and to allow the pin to reliably contactthe side walls of the well. Transfer pin 20 may also have slots, groovesor spirals cut into it to increase volumetric capacity and/or to bettermeter the dispensing action. Transfer pin 20 may be capable of holdingand/or delivering anywhere from 0.1 picoliters to more than 10microliters, but typically holds 0.1 nanoliters to 4 microliters.

FIG. 1 shows an embodiment of the transfer pin 20 having a tapered tipwith a tapered slot that holds the liquid sample. In such an embodiment,the tapered end is small enough to fit inside the well 12, but theoverall pin diameter is still larger than the diameter of the well. Inthe embodiment shown, the tapered end of the transfer pin 20 forms a 40degree angle, and the tapered slot within this end forms a 14 degreeangle. This transfer pin 20 holds adequate amounts of sample (˜0.5 μl),facilitates wicking of the sample to the tip of the pin, and can fillmultiple wells 12 in succession without replenishment. In an alternativeembodiment, the transfer pin 20 is a simple stainless steel dowel with aslot in the end.

Transfer pin 20 may be free to move perpendicular to the surface 14 ofthe platen 10, but movement may be constrained in a plane parallel tothe surface; this implementation is referred to as a floating pin.However, alternative embodiments of the invention may also beimplemented with fixed transfer pins 20 as well. It is generallydesirable to achieve good contact between the transfer pin 20 and thetarget area, but not to damage the target receptacle, well 12. Thisobjective may be achieved by using a floating model transfer pin 20.Floating gravity-fed or spring-loaded transfer pins 20 help withreliable positioning of multiple pins to properly contact correspondingwells 12 to overcome minor errors in alignment. In some embodiments,spring-loaded transfer pins 20 may be used, preferably with “soft”springs having a spring constant that allows for relatively largedisplacement with a small applied force. In other embodiments,gravity-fed floating transfer pins 20 may be more advantageous inapplying minimum force to a target well 12. However, gravity-fedtransfer pins 20 may occasionally stick in one position following asample dispensing cycle. One solution to this problem is to use apressure or vacuum manifold to assist with pin positioning, such as avacuum manifold that sucks the transfer pin 20 back into positionbetween dispensing cycles. Floating transfer pins 20 may also usemagnetism or electro-magnetism for pin positioning, such as use of astrong magnetic field for uniformly extending pins, use of magneticpins, or by accelerating and rapidly decelerating individual pins or theentire array.

Typically, transfer pin 20 is loaded with a liquid sample for transferto platen 10. In typical embodiments, the sample liquid may be anaqueous, DMSO, dimethylformamide (DMF), or acetonitrile solution. Then,transfer pin 20 is moved to a position over the well 12 to be loaded.The transfer pin 20 is lowered until contact is made with the opening ofthe well 12. When the tip of transfer pin 20 is tapered, as shown inFIG. 1, there is maximal contact between the outer surface of the pinand the surface of the interior walls 42 of well 12. Such maximalcontact between pin tip and well wall is desirable because the sampleliquid held in the transfer pin 20 needs to contact the interior wall 42of the well 12 in order for transfer from the pin to be initiated.Furthermore, a tapered pin tip can correct for slight errors in pinplacement with respect to the wells, as the taper of the transfer pin 20guides it into the exact desired position.

Once the transfer pin 20 is positioned in contact with the opening ofwell 12, a portion of the liquid in the pin will be wicked by capillaryaction into the well 12 (and displace any immiscible liquid which maypreviously have been stored therein). The volume of liquid sample thatis transferred is self-metered by the volume of the well 12, and subjectto other environmental variables, such as the action of the layers ofhydrophilic and hydrophobic materials, whether the target area is underan immiscible fluid, and if so, the height of the immiscible fluid overthe target area, the duration of contact with the area, the speed ofwithdrawal from the area, and various of the other variables listedabove with respect to pin transfer.

Initializing the wicking action and wetting the interior walls 42 of thewell 12 is an important point in the transfer process. Occasionally, fora variety of reasons, not all of which are well understood, there willbe difficulty establishing this wicking flow. Embodiments of the presentinvention are directed at overcoming such difficulties in initiating thetransfer of a liquid sample to a storage receptacle by applying anelectric potential. Although this approach may be useful for non-polarliquids, it is especially useful for transferring samples of polarliquids such as aqueous, DMSO, dimethylformamide (DMF), or acetonitrilesolutions can be transferred into a target well 12 by contacting atransfer pin 20 filled with sample and applying a high voltage with lowcurrent (typically less than 5 microamps).

Embodiments of the present invention use the existing transfer pin andplaten well arrangement described with respect to FIG. 1 above, and adda high voltage potential to the transfer pin 20, or at least the tip ofthe pin. Such an arrangement differs from that described in the '815patent in that it avoids the need for a complex plate insulationarrangement (as shown in its FIG. 1), and it does not use theelectrokinetic relationship of voltage-time to volume transferred. Inembodiments where transfer pin 20 is in direct contact with thereceptacle target area, the electric charge applied to transfer pin 20is not directly related to the duration of the sample transfer or theamount dispensed. That purpose is accomplished by hydrophilic attractionof the interior walls 42 and the self-metering action of the platenwells 12. Rather the electrical charge on the transfer pin 20 serves asan activation energy that excites the liquid held by the pin toencourage the wetting of a liquid bridge flow channel between thetransfer pin 20 and the interior walls 42 of well 12. The amount ofsample that is dispensed in a specific embodiment is dependent upon amultitude of variables such as pin geometry, pin coating, sample surfacetension, wetted depth, speed of transfer, sample viscosity, sampleconductivity, the concentration of particles in the sample, voltagelevel, voltage duration, voltage frequency, and loading environment(e.g., air vs. under liquid). Careful control of these variables isrequired. In some embodiments, it may be useful to apply the voltage tothe well 12 rather than to the transfer pin 20.

The voltage necessary to effect sample transfer depends on the physicalproperties of the sample and the receptacle, i.e., well 12, includingtheir affinity for each other. In addition, the choice between AC and DCvoltage supplies may affect the voltage necessary for transfer of aliquid sample, but both types of supplies are acceptable. Generally, thevoltage will be between 10V and 50 kV, typically in the range of 100V to5 kV. The choice of voltage level is affected by effects ofohmic-related heating and material breakdown characteristics. With ahigh dielectric constant liquid, a high voltage of large voltage pulsemay be applied without electrical breakdown.

It is desirable to limit the current flowing from the transfer pin 20 inorder to prevent electrical heating, etching and ionization of thesample, receptacle, pins, air, or immiscible fluid. Therefore, it isimportant to use a high-voltage, low current system. Exampleshigh-voltage, low current sources include a Van De Graaf generator, or astandard high voltage source in series with a high-voltage,high-resistance resistor.

In one specific embodiment, the voltage is applied to the transfer pin20 after it is positioned at the opening of the desired well 12, and thevoltage is removed after the sample has been transferred to the well 12and the transfer pin 20 has been withdrawn from the opening of the well12. In other embodiments, the voltage may be applied to the transfer pin20 before it is positioned at the opening of the desired well 12, andthe voltage is removed after the sample has been transferred to the well12 but before the transfer pin 20 has been withdrawn from the opening ofthe well 12.

In addition, voltage aided sample transfer in various embodiments may bebased on either full, partial, or no physical contact between thetransfer pin 20 and the target well 12. That is, in some embodiments,the end of the transfer pin 20 may be brought into substantial physicalcontact with a portion of the target well 12 in order to transfer aliquid sample from the pin to the well. In other embodiments, thetransfer pin 20 approaches the opening of the target well 12 withoutactually establishing significant contact in order to transfer a liquidsample from the pin to the well. Some embodiments with or withoutcontact may benefit from electrospray effect to transfer a sample fromthe transfer pin 20 to the target well 12.

In various embodiments, either the target well 12 or the entire platen10 may be electrically grounded. In other embodiments, the platen 10 andwell 12 may be ungrounded. Either approach may be successful so long asthere is an appropriate voltage difference between the transfer pin 20and the target well 12. In addition, the platen 10, itself, may be madeof conductive material, or non-conductive material. Moreover, specificembodiments may not necessarily require a combination of hydrophilic andhydrophobic materials as described with respect to FIG. 1, but may beable to exploit the invention using receptacle structures without anysignificant hydrophobic or hydrophilic characteristics, or in ones withall hydrophobic or all hydrophilic materials.

The efficiency of voltage aided sample transfer also may depend on therelative geometries of the transfer pin 20 and the target well 12. Forexample, a transfer pin 20 with a tapered point such as shown in FIG. 1,may be more effective than a different shaped end such as a flat one. Inone specific embodiment in which the well 12 is 280 microns in diameter,a pointed pin tip of less than 200 microns, e.g., 140 microns, may bemost effective. In some specific embodiments, a blunt pin tip also maywork, but in other embodiments, such as under dense fluids, a blunt pintip without a sufficient point on its end may not be operable in avoltage aided transfer arrangement since the liquid may climb the sidesof the transfer pin 20.

In addition to use of an individual transfer pin 20 as shown in FIG. 1,an embodiment may be based on a multiple pin array 30, such as the oneshown in FIG. 2, which is designed so that each transfer pin 20 isspaced to address a unique well 12 in the platen 10. In FIG. 2, multipletransfer pins 20 are held in an array by an electrical insulating plate32. The bottoms of the transfer pins 20 may be slotted as shown in FIG.2, or have some other geometry for holding liquid samples fordispensing. In addition, the bottoms of the transfer pins 20 may besquared off as shown in FIG. 2, or may be tapered as in FIG. 1, or havesome other shape geometry.

The top side of each of the transfer pins 20 may be electricallyconnected either directly or via a resistor, switch, or transistor to avoltage source. The voltage may be specific for each transfer pin 20, ormultiple transfer pins 20 may share a common voltage source.

The top sides of the transfer pins 20 are electrically connected to pinvoltage sources 36 in a voltage control array 34, which may optionallyinclude a voltage control port 38 addressable by an external processor.Each individual pin voltage source 36 may be, for example, a resistorelement in a resistor network (i.e., the voltage control array 34)connected to a high voltage source so that each transfer pin 20 isconnected via its own resistor to the high voltage source. To reduce thecost and size of the system, a single source resistor may be placedbetween the high voltage source and the resistor network, which allowsthe use of smaller, cheaper lower resistance resistors in the networktogether with a single bulky, more expensive, high-resistance resistorat the source. For example, the source resistor could be a 1 to 10gigohm resistor, and the pin resistors could be 1 to 10 megohms each.However, it may be advantageous in terms of uniformity of transferthroughout the pin array 30 to have a higher resistance on the pinresistors, for example each pin having a gigohm resistor.

To individually actuate at least one transfer pin 20 using voltageapplication, a controllable switch may be placed in series with eachactuable pin. These switches may be, for example, high voltagetransistors or relays, and also may be controlled by a microprocessor.In one specific embodiment, each spring-loaded transfer pin 20 may beloaded on a spring, which also acts as an electrical contact to aprinted circuit board voltage control array 34. The printed circuitboard voltage control array 34 may contain the resistor network andconnections to the high voltage source. In some embodiments, the printedcircuit board voltage control array 34 also may contain the switchnetworks and connections to the computer or other device for selecting asample dispensing pattern.

Thus, in one embodiment, each transfer pin 20 in a multiple pin array 30is individually addressable for purposes of applying a high voltagepotential to the pin. In such a pin array 30, one transfer pin 20 at atime may be actuable, multiple pins may be actuable at one time, or allof the pins in the array may be actuable at one time. The more transferpins 20 that are actuated at any one time, the greater the parallelprocessing of the system. By actuating different patterns of multipletransfer pins 20 (in a manner analogous to an ink jet computer printer)patterns of samples may be developed. By repeating this process, layeredpatterns may be developed, including the synthesis of organic moleculessuch as peptides, small molecules or oligonucleotides.

In another embodiment, a pin array may be equipped with a controller forselectively extending or retracting a subset of transfer pins 20 tocause contact or removal from contact of those pins for the purpose ofdispensing a pattern of sample. For example, an array of solenoids couldbe used to retract those transfer pins 20 that are not desired tocontact the receptacle well 12. The solenoids may act directly on thetransfer pin 20, or by a remote drive mechanism such as an array ofpistons positioned slidably in an array of tubes. Alternatively, anarray of controllable valves connected to a vacuum or pressure manifoldmay be used to selectively retract or extend a subset of transfer pins20. Moving the pins in the array 30 so that only transfer pins 20selected for sample transfer approach the opening of selected wells 12avoids inadvertent transfer of a liquid samples from non-selected pinsto non-selected wells, such as by wetting, which may occur even when novoltage is applied to a non-selected pin. It may be desirable to bothselectively actuate a pattern of transfer pins 20 using both movementcontrollers and application of high voltage to the selected pins inorder to prevent inadvertent dispensing, such as by electrospray.

Although various exemplary embodiments of the invention have beendisclosed, it should be apparent to those skilled in the art thatvarious changes and modifications can be made which will achieve some ofthe advantages of the invention without departing from the true scope ofthe invention.

What is claimed is:
 1. A microfluidic liquid dispenser for an assaysystem, the dispenser comprising: at least one transfer pin fortransferring a microfluidic sample of liquid to a target receptacle; anda pin tip at one end of the transfer pin structured to cooperate with anopening in the target receptacle, the pin tip having a high voltagepotential for transferring the sample from the at least one transfer pinto the target receptacle.
 2. A liquid dispenser according to claim 1,wherein the target receptacle is a through-hole well in a platen arrayof wells.
 3. A liquid dispenser according to claim 1, wherein the targetreceptacle is a closed-ended well in a platen array of wells.
 4. Aliquid dispenser according to claim 1, wherein the target receptacleincludes hydrophilic walls regions that attract the sample.
 5. A liquiddispenser according to claim 1, wherein the target receptacle includesan opening hydrophilic region surrounded by hydrophobic material.
 6. Aliquid dispenser according to claim 1, further comprising: a transferpin array including a plurality of transfer pins for transferring aplurality of samples to a corresponding plurality of target receptacles.7. A liquid dispenser according to claim 6, wherein individual transferpins in the array are sequentially actuable.
 8. A liquid dispenseraccording to claim 6, wherein at least one transfer pin in the array isindependently positionable for alignment with respect to the opening ofa target receptacle.
 9. A liquid dispenser according to claim 6, whereinat least one individual transfer pin in the array is gravity-fedfloating.
 10. A liquid dispenser according to claim 1, wherein themicrofluidic sample is from 0.2 to 100 nanoliters.
 11. A liquiddispenser according to claim 1, wherein the transfer pin has a diametergreater than the opening of the target receptacle.
 12. A liquiddispenser according to claim 1, wherein the sample is a polar liquid.13. A liquid dispenser according to claim 12, wherein the polar liquidis an aqueous, DMSO, dimethylformamide (DMF), or acetonitrile solution.14. A liquid dispenser according to claim 1, wherein the high voltagepotential is between 100V and 5 kV.
 15. A liquid dispenser according toclaim 1, further comprising: a voltage control module for controllingwhen the high voltage potential is applied to and removed from the pintip.
 16. A liquid dispenser according to claim 15, wherein the voltagecontrol module operates to apply the high voltage potential to the pintip before the transfer pin is positioned at the target receptacle, andto remove the high voltage potential after the transfer pin is movedaway from the target receptacle.
 17. A liquid dispenser according toclaim 15, wherein the voltage control module includes a resistornetwork.
 18. A liquid dispenser according to claim 15, wherein thevoltage control module includes a controllable switch in series with thetransfer pin.
 19. A liquid dispenser according to claim 1, wherein theat least one transfer pin is able to dispense multiple samples withoutreplenishment.
 20. A method for use in dispensing a microfluidic sampleof a liquid, the method comprising: providing at least one transfer pinfor transferring a microfluidic sample of liquid to a target receptacle,one end of the transfer pin having a pin tip structured to cooperatewith an opening in the target receptacle; and applying a high voltagepotential between the pin tip and the target receptacle for transferringthe sample from the at least one transfer pin to the target receptacle.21. A method according to claim 20, wherein the target receptacle is athrough-hole well in a platen array of wells.
 22. A method according toclaim 20, wherein the target receptacle is a closed-ended well in aplaten array of wells.
 23. A method according to claim 20, wherein thehigh voltage potential is applied to the transfer pin.
 24. A methodaccording to claim 20, wherein the high voltage potential is applied tothe target receptacle.
 25. A method according to claim 20, wherein thetarget receptacle includes hydrophilic walls that attract the sample.26. A method according to claim 20, wherein the target receptacleincludes an opening surrounded by hydrophobic material.
 27. A methodaccording to claim 20, further comprising: providing a transfer pinarray including a plurality of transfer pins for transferring aplurality of samples to a corresponding plurality of target receptacles.28. A method according to claim 27, wherein individual transfer pins inthe array are sequentially actuable.
 29. A method according to claim 27,wherein at least one transfer pin in the array is independentlypositionable for alignment with respect to the opening of a targetreceptacle.
 30. A method according to claim 27, wherein at least onetransfer pin in the array is gravity-fed floating.
 31. A methodaccording to claim 20, wherein the microfluidic sample is from 0.2 to100 nanoliters.
 32. A method according to claim 20, wherein the transferpin has a diameter greater than the opening of the target receptacle.33. A method according to claim 20, wherein the sample is a polarliquid.
 34. A method according to claim 33, wherein the polar liquid isan aqueous, DMSO, dimethylformamide (DMF), or acetonitrile solution. 35.A method according to claim 20, wherein the high voltage potential isbetween 100V and 5 kV.
 36. A method according to claim 20, furthercomprising: controlling when the high voltage potential is applied toand removed.
 37. A method according to claim 36, wherein the controllingstep includes applying the high voltage potential before the transferpin is positioned at the target receptacle, and removing the highvoltage potential after the transfer pin is moved away from the targetreceptacle.
 38. A method according to claim 36, wherein the controllingstep uses a resistor network.
 39. A method according to claim 36,wherein the controlling step uses a controllable switch in series withthe transfer pin.
 40. A method according to claim 20, furthercomprising: applying evaporation control measures to the targetreceptacle.
 41. A method according to claim 40, wherein the applyingstep includes immersing the target receptacle in an immiscible liquid.42. A method according to claim 41, wherein the immiscible liquid is aperfluorinated hydrocarbon, hydrocarbon, or silicone fluid.
 43. A methodaccording to claim 40, wherein the applying step uses at least one ofhumidity control, fluid pressure, and receptacle cooling.
 44. A methodaccording to claim 20, wherein the applying step includes positioningthe transfer pin in direct contact with target receptacle.
 45. A methodaccording to claim 20, wherein the applying step includes positioningthe transfer pin near the target receptacle without direct contact. 46.A method according to claim 20, further comprising: sequentiallytransferring multiple samples to the target receptacle to produce alayered pattern of samples.
 47. A method according to claim 20, whereinthe at least one transfer pin is able to dispense multiple sampleswithout replenishment.
 48. A microfluidic assay system comprising: atleast one liquid sample storage device including a plurality of storagereceptacles; and a microfluidic liquid dispenser having: i. a highvoltage supply that develops a high voltage potential; ii. at least onetransfer pin for transferring a microfluidic sample of liquid to atarget storage receptacle, one end of the transfer pin having a pin tipstructured to cooperate with an opening in the target storagereceptacle; and iii. a voltage controller for applying the high voltagepotential from the high voltage supply between the pin tip and thetarget storage receptacle for transferring the sample from the at leastone transfer pin to the target storage receptacle; and a dispenserpositioning module that positions the liquid dispenser to enable thetransfer pin to cooperate with the target receptacle for transferringthe sample.
 49. An assay system according to claim 48, wherein thestorage device is a platen array of through-hole wells.
 50. An assaysystem according to claim 48, wherein the storage device is a platenarray of closed-ended wells.
 51. An assay system according to claim 48,wherein the voltage controller applies the high voltage potential to thetransfer pin.
 52. An assay system according to claim 48, wherein thevoltage controller applies the high voltage potential to the targetstorage receptacle.
 53. An assay system according to claim 48, whereinthe target storage receptacle includes hydrophilic walls that attractthe sample.
 54. An assay system according to claim 48, wherein thetarget storage receptacle includes an opening surrounded by hydrophobicmaterial.
 55. An assay system according to claim 48, wherein the liquiddispenser includes a transfer pin array including a plurality oftransfer pins for transferring a plurality of samples to a correspondingplurality of target storage receptacles.
 56. An assay system accordingto claim 55, wherein individual transfer pins in the array aresequentially actuable.
 57. An assay system according to claim 55,wherein at least one transfer pin in the array is independentlypositionable for alignment with respect to the opening of a targetstorage receptacle.
 58. An assay system according to claim 55, whereinat least one transfer pin in the array is gravity-fed floating.
 59. Anassay system according to claim 48, wherein the microfluidic sample isfrom 0.2 to 100 nanoliters.
 60. An assay system according to claim 48,wherein the transfer pin has a diameter greater than the opening of thetarget storage receptacle.
 61. An assay system according to claim 48,wherein the sample is a polar liquid.
 62. An assay system according toclaim 61, wherein the polar liquid is an aqueous, DMSO,dimethylformamide (DMF), or acetonitrile solution.
 63. An assay systemaccording to claim 48, wherein the high voltage potential is between100V and 5 kV.
 64. An assay system according to claim 48, wherein thevoltage controller applies the high voltage potential to the pin tipbefore the transfer pin is positioned at the target storage receptacle,and removes the high voltage potential after the transfer pin is movedaway from the target storage receptacle.
 65. An assay system accordingto claim 48, wherein the voltage controller uses a resistor network. 66.An assay system according to claim 48, wherein the voltage controlleruses a controllable switch in series with the transfer pin.
 67. An assaysystem according to claim 48, wherein the storage device usesevaporation control measures to control evaporation of samples from thestorage receptacles.
 68. An assay system according to claim 67, whereinthe evaporation control measures include immersing the storagereceptacles in an immiscible liquid.
 69. An assay system according toclaim 68, wherein the immiscible liquid is a perfluorinated hydrocarbon,hydrocarbon, or silicone fluid.
 70. An assay system according to claim67, wherein the evaporation control measures include at least one ofhumidity control, fluid pressure, and receptacle cooling.
 71. An assaysystem according to claim 48, wherein the positioning module positionsthe dispenser so that the at least one transfer pin makes direct contactwith target storage receptacle for transferring the sample.
 72. An assaysystem according to claim 48, wherein the positioning module positionsthe dispenser so that the at least one transfer pin is near the targetstorage receptacle without direct contact for transferring the sample.73. An assay system according to claim 48, wherein the liquid dispenseroperates to sequentially transfer multiple samples to the target storagereceptacle to produce a layered pattern of samples.
 74. An assay systemaccording to claim 48, wherein the at least one transfer pin is able todispense multiple samples without replenishment.